1. Field of the Invention
The present invention relates to superconductor materials.
2. Description of the Related Art
Superconductor materials are gaining ever increasing attention for their ability to carry significantly large currents without resistance. Even at high frequencies well into the microwave regime and at large current levels, these materials can exhibit negligible dissipation. The so-called high temperature superconductors are especially important for many applications because they can exhibit such properties at temperatures of 77 K or higher. One promising application for these materials is in the form of epitaxially grown superconductor thin films for use in wireless communication systems, both satellite and ground based.
The high temperature superconductors are generally anisotropic oxide materials. In a crystal of a typical high temperature superconductor, say YBa.sub.2 Cu.sub.3 O.sub.7-.delta., currents are readily carried in the `a` or `b` crystallographic directions while the `c` direction can only sustain a small current without significant dissipation. Many other high temperature superconductor materials are even more anisotropic. As a result, optimal current-carrying capacity in a film requires an orientation of the `c` axis everywhere perpendicular to the substrate. This geometry enables the current to flow in the `a` and `b` directions only. Even with the proper alignment of the `c` axis, the alignment of the `a` and `b` directions is relevant for the current-carrying capacity. In some applications, it may be preferable for the `a` and `b` directions to be consistent throughout the film, although for YBa.sub.2 Cu.sub.3 O.sub.7-.delta., and many other high temperature superconductors, this may lead to different properties in the `a` and `b` directions. Large currents can also be carried if the `a` and `b` directions occasionally interchange via a mechanism called "twinning". It is generally considered not desirable to have other relative orientations of the `a` and `b` directions in different parts of the film since these lead to large angle grain boundaries which are found to decrease the current carrying capacity of the film.
Typically, as these films are grown their outer surface tends to roughen. This can be due to particulates attaching to the film during growth or the nucleation of undesired orientations. Even if such difficulties are avoided the surface will tend to roughen as it grows and can be characterized by a series of peaks and valleys. This is usually attributed to a "spiral growth mode" known to be typical for these materials. In such films, regardless of the height of the peaks, the current is limited by the thickness in the valleys. Further, any current carried or induced near the surface of a peak must necessarily travel in the `c` axis direction to pass through a valley.
Moreover, attempts to continue the growth process and increase the useful thickness have progressively diminishing returns thereon since the peaks tend to gain height at the expense of the valleys. In other words, the valleys do not see a commensurate increase in height.
In addition to reducing the current carrying capacity, rough films have other undesirable properties. These include increased microwave surface resistance and increased electrical noise. Such defects in the films will also make patterning the film difficult and hamper the film development of more complicated multi-layer structures on top of the superconducting film.
It is an object of the present invention to provide novel superconductors.